<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    
</body>
<script>

    eval( 'var num = 123;' ); // 就好像此时代码中写了 var num = 123 一样 

    console.log( num );

</script>
</html>